Starting today, it was impossible to get the profile up, I was presented with a blank page.
for the moment, my workaround was to install thunderbird on my PC, click on troubleshoot, then about:profiles, and finally create a new profile by pointing to my exiting one, copied from the portable installation, and making that the default. (the portable application has to be closed at that point, or it won't allow you access).
I couldn't direct to the portable profile from the portable app, it defaults to local appdata and won't redirect to the portable.
Sorry for the garbled explanation. I'm hoping it will save someone a couple of hours frustration.
And panic 
.
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I am having the exact same problem.
Here's a screenshot:
https://i.imgur.com/B4NOLtO.png
I can see several threads here recently reporting the same problem. I have this issue too.
I use Thunderbird on my C drive.
I have another Thunderbird on my Z drive
Both for different email accounts that i need to keep separate.
The C drive instance has been without issue.
The Z drive instance has been having this issue of loading up blank.
Fortunately, I have back up copies.
I copied the backed up Z drive version to my desktop and everything worked fine.
I noticed that in Help > Troubleshooting Information > about:profiles contain 3 entries.
Which I removed and everything still worked fine.
I then copied this working instance from my desktop back to my Z drive and on opening it - back to blank page/no folders/no emails etc.
Never had this issue before, tried updating and rolling back versions and same issue occurs again.